[NFC][libc++][test][AIX] fix SIMD test XFAIL for clang before 19 (#94509)

058e4454 added an XFAIL for this test on AIX because of a backend
limitation. That backend limitation
has been resolved by 0295c2ad and will be available for clang 19, so we
should update the test to
limit the XFAIL to clang versions before that.